Add MCP Apps Overview document

Adds `docs/overview.md` covering the core concepts: architecture,
lifecycle, UI resources, bidirectional communication, theming, display
modes, and security. Includes mermaid flowchart and sequence diagrams.

Also adds the `@boneskull/typedoc-plugin-mermaid` TypeDoc plugin for
mermaid diagram support. However, the plugin handles HTML entities from
TypeDoc incorrectly, so `scripts/typedoc-plugin-fix-mermaid-entities.mjs`
decodes HTML entities before the plugin processes them.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/typedoc-plugin-fix-mermaid-entities.mjs

@@ -0,0 +1,54 @@
+/**
+ * TypeDoc plugin that properly decodes HTML entities in mermaid code blocks.
+ *
+ * This runs BEFORE the mermaid plugin and converts HTML entities back to raw
+ * characters, allowing mermaid to parse them correctly.
+ *
+ * The @boneskull/typedoc-plugin-mermaid converts < to #lt; and > to #gt;,
+ * but those aren't valid mermaid entities (mermaid uses numeric codes like #60;).
+ * This plugin sidesteps the issue by decoding entities before mermaid sees them.
+ */
+
+import { Renderer } from "typedoc";
+
+/**
+ * Decode HTML entities back to raw characters.
+ * @param {string} html - HTML-encoded string
+ * @returns {string} Decoded string
+ */
+function decodeHtmlEntities(html) {
+  return html
+    .replace(/&#(\d+);/g, (_, num) => String.fromCharCode(parseInt(num, 10)))
+    .replace(/&#x([0-9a-fA-F]+);/g, (_, hex) =>
+      String.fromCharCode(parseInt(hex, 16)),
+    )
+    .replace(/&lt;/g, "<")
+    .replace(/&gt;/g, ">")
+    .replace(/&quot;/g, '"')
+    .replace(/&#39;/g, "'")
+    .replace(/&amp;/g, "&"); // Must be last
+}
+
+/**
+ * TypeDoc plugin entry point.
+ * @param {import('typedoc').Application} app
+ */
+export function load(app) {
+  // Use high priority (200) to run before the mermaid plugin (default is 0)
+  app.renderer.on(
+    Renderer.EVENT_END_PAGE,
+    (page) => {
+      if (!page.contents) return;
+
+      // Find mermaid code blocks and decode HTML entities
+      page.contents = page.contents.replace(
+        /<code class="mermaid">([\s\S]*?)<\/code>/g,
+        (match, code) => {
+          const decoded = decodeHtmlEntities(code);
+          return `<code class="mermaid">${decoded}</code>`;
+        },
+      );
+    },
+    200,
+  );
+}